Silicone rubbers are used in the electrotechnical industry as electrical insulation for indoor and outdoor applications such as, for example, hollow core insulators, cable accessories, medium and high voltage surge arresters or vacuum interrupters. Both high temperature vulcanizing silicone rubbers (HTV-SR) and liquid silicone rubbers (LSR) are used for these applications. In the case of LSR, the material can be composed of two different components A and B, which are mixed together and afterwards thermally cured. Curing time plays a role in the total production time of these products and can vary between 15 minutes for small insulators up to 40 minutes or more for large insulators.
A faster way of producing electrical insulations is to use room temperature vulcanizing (RTV) silicone rubbers, including room temperature vulcanizing liquid silicone rubbers (RTV-LSR), which cure in the presence of ultraviolet radiation (UV). For example, the UV-curing of such UV-vulcanizable RTV silicone rubber compositions does not require or employ a heating step, and can occur within seconds to minutes. The use of UV vulcanizable RTV silicone rubber compositions, including room temperature vulcanizing liquid silicone rubbers, allows for the significant reduction of production times and therewith also production costs.
With regard to electrical applications of such UV-curable RTV silicone rubber compositions, it can be desirable for there to exist a strong adhesion between the cured silicone rubber composition and the substrate. If the adhesion is low, electric discharges can occur on the interfaces, which can lead to an electrical breakdown of the insulation or device.
For thermally cured silicones there exist primer compositions to improve adhesion of the thermally cured silicone rubber to the substrate. Individual primers can be selected depending on the substrate. Such primer compositions can be used in the form of liquid solutions containing compounds selected, for example, from hydrolysable silicates, hydrolysable titanates, reactive silanes and/or siloxanes, optionally together with an organic solvent. When the primer is applied to a substrate surface and is exposed to moist atmosphere at ambient or elevated temperature, a thin solid film is formed on the substrate surface.
In the case of UV-cured silicones, however, it has been shown that primer compositions do not provide sufficient adhesion to the substrate and, therefore, do not work properly, although these primers provide good results when applied together with thermally cured silicones. This is true for substrates such as metals, ceramics, glass, and rubber/glass composites as well as plastic materials. Use of thermally cured silicones can be disadvantageous because it can require either the development of new primer compositions or the introduction of an additional heat curing step into the production. The latter is a disadvantage for the industrial use of UV-cured silicones and limits the time and cost benefits of using UV-curing silicones.